A hybrid electric vehicle (HEV) is a vehicle that is powered by an electric motor and an internal combustion (IC) engine, including diesel or other hydrocarbon or fossil fueled engines. In a diesel-electric series hybrid propulsion system, the diesel engine is not directly linked to the transmission for mechanical drive power. Rather, the energy produced from the IC engine is converted to electric power by a generator, which re-charges a battery in order to provide power to one or more electric motors. The electric motor system provides torque to turn the wheels of the vehicle. Because the IC engine is not directly connected to the wheels, the IC engine can operate at an optimum rate and can be automatically or manually switched off for temporary all-electric, zero-emission operation of the vehicle. Conventional diesel-electric series hybrid propulsion systems are traditionally well-suited for lightweight commuting vehicles and stop-and-go transit buses.
Presently, there are no class 8 DOT over-the-road transportation vehicles such as semi-tractor trucks that use a diesel-electric series hybrid propulsion system as a power source at normal load (weight) conditions which are capable of maintaining highway speeds for an extended period of time. This is due, in part, to the power requirements of the transportation industry and, in part, to present day battery technology which does not meet the demands of class 8 DOT vehicles. Currently, the transportation industry relies on IC engine drive train systems as the propulsion systems for class 8 DOT vehicles in view of the reliability and cost advantage of operating such vehicles with IC engine drive train systems.